Radiotherapy is used to treat 40% of cancer patients in Australia and up to 55% of cancer patients in other countries. Patient motion, from the respiratory, cardiac, musculoskeletal, gastrointestinal and genitourinary systems move the tumour during a radiation treatment, which typically takes 1-30 minutes. Movement of the tumour means a misalignment of the radiation beam and tumour. This misalignment causes a loss of treatment efficacy resulting in a lower chance of tumour control, and an increase in treatment related side effects. Much effort is being put into developing methods to mitigate the effects of tumour translational motion. However, evidence shows that tumour rotation is also deleterious to treatment accuracy.
Image guided radiotherapy (IGRT) using onboard kilovoltage imager is an effective localization method to reduce the uncertainties in the estimation of tumor location and aids in patient positioning. IGRT has also been used for monitoring the target in real-time during radiation delivery. As it is difficult for the 2D kilovoltage (kV) imaging system to image soft tissue, oncologists implant radiopaque markers in the vicinity of the tumor and use the markers as indicators to determine the location of the tumor. Although IGRT is used extensively to position patients for radiotherapy treatment, tumor rotation is not currently explicitly considered during the treatment planning and delivery of radiotherapy.
In addition to prostate tumor translation, prostate tumor rotation during treatment can also affect the target dose coverage. For that reason, a small rotation may cause some part of the tumor to receive a dose that is lower than the prescribed amount. Previous studies on prostate rotation show that rotational errors are significant in some cases. The systematic uncertainty per patient for prostate rotation with 2.30°, 1.56° and 4.13° and the mean random components with 1.81°, 2.02° and 3.09° were estimated. It was also reported that the largest rotational errors occurred around the RL axis, but without preferring a certain orientation. There have been several studies to correct for tumor rotation and to reduce treatment margins for IMRT. These studies have corrected for rotation by moving or rotating the gantry, collimator, and/or couch in real-time or by using a dynamic multi leaf collimator.
What is needed is a method that estimates real-time rotation from x-ray images acquired during radiotherapy by extending translation estimation methods. One such method is to invoke iterative closest point (ICP) algorithm.